(1) Field of the Invention
The present invention is directed to a method of inhibiting scale and corrosion in aqueous systems, especially in cooling water systems and their associated equipment.
(2) Description of the Related Art
This invention relates to a method of inhibiting the scale formation on and the corrosion of metal surfaces, such as iron and iron alloys and, in particular, low carbon steel which are part of an aqueous system. Aqueous systems, include, for example, cooling water and related water-handling equipment, such as cooling towers and associated pumps, heat exchangers and heating systems, gas scrubbing systems and other similar equipment and systems. Problems commonly encountered in these systems include not only the electrochemical corrosion of iron and iron alloys which are in contact with the circulating water but also the precipitation of scale-forming calcium salts. These two problems are, in fact, very closely related because methods commonly used to control on of these problems often aggravates the other.
For many years, the most common method of controlling corrosion in aqueous systems was to treat the water with hexavalent chromium salts, such as sodium chromate. At the same time, scaling was prevented by treating the water with mineral acids to maintain a low enough pH to prevent the precipitation of scale-forming calcium salts. In order to reduce the chromate concentration, phosphate and zinc salts have been combined with the chromates to provide good corrosion control.
Environmental concerns over the discharge of even small amounts of hexavalent chromium has caused new methods to be sought that would provide total corrosion inhibition without its use.
Some of the ways include the use of various combinations of zinc salts, phosphates, polyphosphates, and organic phosphonic acid derivatives and their salts. In particular, the use of high concentrations of orthophosphate is well established, but such high concentrations make it necessary to work in the presence of high dosages of anionic dispersants in order to prevent calcium phosphate from fouling. Another inorganic salt, sodium nitrite, is also well known as a corrosion inhibitor, but it is normally necessary to use it in concentrations of 500-1000 ppm which is environmentally unacceptable.
Organic phosphonates have also been extensively used in the inhibition of scale and corrosion in aqueous systems. The use of these compounds is illustrated by U.S. Pat. Nos. 4,640,818 (aminophosphonic acid plus manganese); 4,692,317 (phosphonohydroxyacetic acid plus cationic polymer); 4,671,934 (aminophosphonic acid/phosphate mixtures); and 4,085,134, 4,212,734 and 4,229,294 (all directed to alkyl phosphonic-sulfonic acids).
It is highly desired to provide a means to readily treat aqueous systems which are susceptible to corrosion and scale formation in a manner which inhibits both of these adverse conditions. It is further desired to provide a means to inhibit corrosion and scale formation under a wide range of water conditions. Finally, it is desired to provide a means to inhibit corrosion and scale formation in an environmentally acceptable manner.